1. Field of the Invention
This invention relates to a crystal pulling method for growing semiconductor crystal using a crucible called an integral type double crucible, and more particularly to a uniform resistivity control method used in a case where two kinds of dopants or two different dopant concentration melts are used.
2. Description of the Related Art
In a case where rod-like semiconductor single crystal is grown from melt in the crucible by Czochralski technique (CZ technique), impurity concentration distribution C in the longitudinal direction of the grown single crystal can be expressed as follows. EQU C=kCo(1-G).sup.k-1
where k is a segregation coefficient of dopant, Co is the initial impurity concentration of the melt, and G is solidification ratio. Therefore, the impurity concentration distribution in the longitudinal direction significantly varies when k is small, thus lowering the yield of the single crystal having a desired preset resistivity range.
In order to solve this problem, a floating type double crucible method in which the surface level of melt in the inner crucible is kept at a constant level has been proposed and used for growing single crystals of germanium and silicon (See J. Applied Physics vol. 9 No. 8, '58, Japanese Patent Publication 60-18634).
Now, the double-crucible method is explained with reference to FIG. 9. As shown in FIG. 9, inner crucible 2 is arranged as a floating crucible inside outer crucible 1, and small hole 3 is formed in the bottom portion of inner crucible 2. When crystal 6 is pulled from melt 4 in inner crucible 2, the balance between the buoyancy and gravitational force of the inner crucible is utilized, for example, or the outer crucible is lifted relative to the fixed inner crucible, so that melt 5 can be supplied from the outer crucible to the inner crucible, thus keeping the height h of the surface level of the melt in the inner crucible at a constant level.
Assume that the impurity concentration of melt 5 in the outer crucible is Co, and the impurity concentration of melt 4 in the inner crucible is Co/k (k is a segregation coefficient). Then, the concentration of impurity taken into pulling crystal 6 becomes Co in the pulling process in which surface level h of the melt is kept constant. Thus, the same amounts of melt (pure silicon or germanium) and impurity as those used for growing the crystal are always supplied from melt 5 in the outer crucible to melt 4 in the inner crucible. As a result, the impurity concentration of melt 4 in the inner crucible is kept at Co/k and, therefore, the impurity concentration of pulling crystal 6 can also be kept at constant value Co.
However, in the pulling process, the melt is consumed. After the bottom portion of inner crucible 2 has reached the inner bottom portion of outer crucible 1, the melt surface level in the inner crucible cannot be kept constant, and the impurity concentration of crystal 6 will vary (increase) as the solidification ratio increases. That is, the impurity concentration can be kept constant only within the following range of the solidification ratio X: EQU 0.ltoreq.X.ltoreq.1-(h/H) (1)
where H is the initial surface level of the melt in the outer crucible, and h is the surface level of the melt in the inner crucible to be kept constant during the pulling process. Therefore, in a case where the floating type double-crucible method is effected by using impurity acting as donor or acceptor to grow crystal having constant resistivity in the longitudinal direction, the resistivity can be kept constant only when the solidification ratio is less than 0.6 to 0.7. If the solidification ratio becomes larger, the resistivity will significantly vary.
Further, it was found that the constant resistivity of the crystal could not be obtained even in the range of the solidification ratio expressed by equation (1) when the floating type double crucible method was effected to be intended to grow crystal having constant and high resistance in its longitudinal direction. This problem occurs when, for example, the low donor (phosphorus P) concentration is used to grow high resistance N-type silicon single crystal with the resistivity of more than 20 .OMEGA..cm and so the donor concentration is not sufficiently high with respect to the acceptor concentration of boron (B), aluminum (Al) or the like flowing out of the quartz crucible. In this case, as shown in FIG. 10, the actual resistivity value (indicated by mark .cndot.) gradually increases with increase in the solidification ratio and the resistivity value of the crystal cannot be determined only by the concentration (indicated by mark o) of doped donor impurity (P).
The inventors of this invention have proposed a pulling apparatus having an integral type double crucible (Japanese Patent Application No. 61-221896, or U.S. patent application Ser. No. 091,947 filed on Sept. 1, 1987). The pulling apparatus is shown in FIG. 1. In FIG. 1, 11 denotes an outer crucible, and 14 denotes a cylindrical separation wall integrally and coaxially formed with outer crucible 11. The inner space of outer crucible 11 is divided into inner chamber 20 and outer chamber 21 by separation wall 14. Inner chamber 20 and outer chamber 21 are connected with each other by means of small hole 15 and narrow pipe-like coupling tube 16 formed with small hole 15. With this construction, melt ML2 in outer chamber 21 is supplied to inner chamber 20 while single crystal 17 is being pulled from melt ML1 in the inner chamber.
The impurity of melt ML1 in the inner chamber is inhibited from flowing into the outer chamber by the preset length L of coupling tube 16 not only during the pulling process but also when melt is not transferred from outer chamber 21 to inner chamber 20. Length L is determined to be larger than four times inner diameter a of tube 16. In this respect, the integral type double crucible is different from the floating type double crucible which has only hole 15.
One of the proposed crystal pulling methods using the integral type double crucible is to solve the problem that the impurity concentration in the longitudinal direction of crystal formed by the floating type double crucible method is limited by the solidification ratio used in equation (1). In this case, doped material melt ML1 (impurity concentration Ci) is received in inner chamber 20 of the integral type double crucible and undoped material melt ML2 is received in outer chamber 21. Further, ratio r/R of radius r of inner chamber 20 and radius R of outer chamber 21 is set to be equal to square root .sqroot.k of segregation coefficient k of the doping impurity and crystal 17 of impurity concentration kCi is pulled by .pi.R.sup.2 .DELTA.H (.DELTA.H is decrease in the surface level of melt ML1) from melt ML1 in the inner chamber. Then, the amount of impurity .pi.R.sup.2 .DELTA.H.times.kCi included in a portion of crystal 17, which has been grown during the surface level decreases by .DELTA.H, becomes equal to the amount of impurity .pi.r.sup.2 .DELTA.H.times.Ci included within a portion of melt ML1 having width .DELTA.H so that the impurity concentration of melt ML1 in the inner chamber is kept at constant value Ci during the pulling process. As a result, the impurity concentration in the longitudinal direction of crystal 17 can be kept at constant value kCi.
FIG. 11 shows the relation between the resistivity (ordinate) and the solidification ratio (abscissa) of single crystal formed by the integral type double crucible method effected in a condition of r/R=.sqroot.k in comparison with those obtained by the CZ technique and the floating type double crucible method. As shown in FIG. 11, in a case where constant-resistivity crystal is formed by the proposed integral type double crucible method (Japanese Patent Application No. 61-221896 or U.S. Ser. No. 091,947), the problem that the solidification ratio is limited by equation (1) in the floating type double crucible method can be solved.
However, even in the integral type double crucible pulling method, electrically conductive impurity (B, Al or the like) flows out of the crucible itself and may cause an inadvertent influence, making it difficult to attain crystal of high and constant resistivity. For example, in a case where two kinds of dopants or two different dopant concentration melts are used, there still remains a problem.